1. Technical Field
The present disclosures relate to an image scanning apparatus using a gray reference member of which a reflection coefficient is lower than that of a white reflection member.
2. Related Art
Generally, an image scanning apparatus employs a white reference member as a light distribution reference member to be used for a shading correction. When scanning is executed, an image of an original sheet passing over the white reference member is scanned. When the white reference member is used, however, there may occur a show-through phenomenon which is a phenomenon that a change in thickness of color on a back surface of the original sheet affects scanning of an image formed on a front surface of the original sheet. In order to reduce such a show-through phenomenon, image scanning apparatuses employing the gray reference member, which has a smaller reflection coefficient than the white reference member, have been suggested recently.
An example of such an image scanning apparatus employs a non-white reference member which is provided as a guide for the original sheet in a sheet conveying device. In such an image scanning apparatus, the shading correction is performed, based on the reflection coefficient of the non-white reference member, so that a reflection density value obtained by scanning the non-white reference member has substantially the same quantity as a reflection density value obtained by scanning the white reference member. When an image on the original sheet, which is conveyed by the conveying device, is scanned, the shading correction is applied based on the reflection density value compensated as above.
When dust or blot is adhered on a reflection surface of the reference member, density values of pixels obtained by scanning a portion of the reflection surface on which the dust or blot is adhered may be largely different with respect to the density values of neighboring pixels. When the light quantity of the light source is adjusted so that the reflection density value obtained by scanning the gray reference member has a predetermined density value, it is necessary to determine whether the reflection density values, or gradation values obtained by scanning the gray reference member includes values which vary relatively largely with respect to the density values (or gradation values) of the neighboring pixels.
When density values for one line, namely gradation values for one line of pixels are obtained by scanning the gray reference member, whether the gradation values fluctuates relatively largely due to the dust/blot may be determined. However, execution of such a determination for all the pixels may result in elongation of scanning period.
In consideration of the above, aspects of the disclosures provide an improved image scanning apparatus capable of obtaining accurate gradation values when the gray reference member is scanned, without elongating the time period for scanning.
According to aspects of the disclosures, there is provided an image scanning apparatus which is provided with a gray reference member arranged in a conveying path in which an original sheet is to be conveyed, the gray reference member having a reflection coefficient smaller than that of a white color, a scanning unit configured to scan an image on the original sheet on a line basis, the scanning unit including a light source configured to illuminate the original sheet when passing the gray reference member and a plurality of photoelectric conversion elements aligned in a scanning direction which is a transverse direction of the conveying path, a signal conversion unit configured to convert analog signals from the plurality of photoelectric conversion elements to digital signals, respectively, a storage and a controller. The controller is configured to set first to N-th target numbers of pixels each representing the number of a plurality of pixels, N being more than two, changeably set a reference gradation value which represents a value of a white signal output by the signal conversion unit for each pixel when the light source illuminates the gray reference member, between a highest gradation value and a smaller gradation value which is smaller than the highest gradation value, store, in the storage, a specific reference gradation value in association with the target number of pixels when the number of pixels having gradation values equal to or greater than the specific reference gradation value is a specific target number of pixels, determine a correction target number which is a minimum target number of pixels from among a plurality of target numbers of pixels respectively providing differences, which is less than a predetermined value, between the reference gradation value stored in association with an M-th target number among the first to N-th target numbers and the reference gradation stored in association with an (M+1)-th target number, determine an anomaly pixel number which is the number of anomaly pixels having gradation values which are equal to or greater than a reference gradation value, the reference gradation value being determined based on the reference gradation value stored in the storage in association with the correction target number of pixels from among the gradation values of the with signal output by the signal conversion unit for respective pixels when the light source illuminates the gray reference member, and correct the gradation values of the anomaly pixels, the number of the anomaly pixels of which gradation values are to be corrected being the anomaly pixel number.
It is noted that the scanning unit may be configured to scan an image on only one surface of the original sheet or images on both sides of the original sheet. In the latter case, two scanning units may be arranged along the sheet conveying path to face the original sheet from opposite directions.
According to aspects of the disclosures, as far as the base gradation value is determined based on the reference gradation value which is stored in association with the target number of pixels, a method of determining the reference gradation value is not limited to a specific one. For example, the base gradation value may determined to have the same value of the reference gradation value which is stored in association with the target number of pixels, or a value obtained by adding a predetermined value to the reference gradation value.
According to aspects of the disclosures, correction of the gradation values corresponding to the anomaly pixels due to dust or the like can be made in various ways. For example, the gradation value of an anomaly pixel may be replaced with the gradation value of the neighboring pixel(s), or replaced with an average of the gradation values of the white signal output by the signal conversion unit for respective pixels of one line.